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Yes, a parallel connection is proper one. Therefore, Your calculation of current drawn is also correct.

I don't see any strong need for calculation of the battery voltage. I'd rather consider calculation of battery capacity, as it will affect your working time, thus changing possible range of Your robot. I'd suggest using LiPo or LiIon batteries - these types of batteries offer slightly lower voltage than 4.5V, but have great energy density.

Bonus question: Yes, You can double Your H-bridge current by bridging its outputs. This is often used in minisumo category robots to drive more powerful motors. To do this, you should also bridge correct input pins: A_PWM with B_PWM, A_IN1 with B_IN1 and A_IN2 with B_IN2.

$\begingroup$Thanks - as for battery voltage, I wasn't sure if grouping motors together like this would increase the voltage requirements. I understand about the capacity (Ah) need being greater for 6 vs 4 vs 2 (at the voltage).$\endgroup$
– Carol AndorMarten LiebsterJul 18 '18 at 12:34